Technologies for growing high quality bulk cadmium telluride (CdTe), which may be used for fabricating photovoltaic (PV) devices, have significantly improved in recent years. Reductions in the defect density throughout the bulk CdTe material have led to reduced carrier recombination rates within the material, so that thinner devices may now render a high value of open-circuit voltage (Voc) once only achievable in thicker devices. Further, the combination of thinner CdTe materials and reduced bulk recombination rates means that minority carrier lifetimes (MCLs), such as measured by time-resolved photoluminescence (TRPL) measurements, are now long enough for minority carriers to reach the back surface of the CdTe material where back surface contacts are formed. For example, CdTe PV devices today may have CdTe layers on the order of 3-μm or less, and MCLs on the order of 1-2 ns. The advancements have been sufficient for device efficiency to increase from less than 10% to greater than 20%. For those devices with an efficiency greater than 20%, it is very likely that minority carriers are reaching the back contact, where detrimental surface recombination can occur. If they recombine at the back contact, the recombination of minority carriers may add to the Jo (dark current) and reduce the Voc, thus reducing the efficiency of the device. Further, to continue to reduce the cost of production modules, the CdTe layer in the device needs to continue to get thinner, and the MCLs need to continue to get longer. As a result, the surface recombination velocity, which formerly contributed to only a negligible fraction of the overall recombination rate in a CdTe device, may soon become the dominant recombination mechanism that limits what open-circuit voltage Voc can be achieved.
As presently known in the art, CdTe material at the back surface is treated to make the stoichiometry of the surface material Tellurium (Te) rich before a contact layer (that includes copper, for example) is deposited onto this Te-rich layer, and often diffused into the bulk of the CdTe material using heat. This is a well-known back surface contact formation process where the contact layer can be chosen to establish an acceptable minority-carrier barrier at the back surface contact. Nonetheless, now that more minority carriers are reaching the back surface contact layers, such treatment processes may no longer provide sufficient surface passivation to adequately address surface recombination. TRPL measurements show that high surface recombination at these back surface contact layers is emerging as a leading challenge to further improving minority carrier lifetimes in CdTe devices. Surface recombination will soon be, if it is not already, one of the primary limiting factors preventing further increases in CdTe PV device efficiency. Accordingly, it would be advantageous to provide new methods for surface passivation that reduce surface recombination in CdTe material.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.